This invention relates to array antenna systems and particularly to such systems wherein the antenna element pattern is modified by providing a coupling network between the antenna input ports and antenna elements, so that the effective element pattern associated with each input port is primarily within a selected angular region of space.
An array antenna system may be designed to transmit a desired radiation pattern into one of a plurality of angular directions in a selected region of space. In accordance with the customary design of array antennas, each of the antenna elements has an associated input port and by variation of the amplitude and/or phase of the wave energy signals supplied to the input ports, the antenna pattern can be electronically steered in space to point in the desired radiation direction or otherwise controlled to radiate a desired signal characteristic, such as a Doppler pattern. When it is desired to have an array antenna radiate its beam over a selected limited region of space, it is preferable that the radiation pattern of the individual antenna elements also be primarily within the selected angular region. This permits maximum element spacing while suppressing undesired grating lobes. Control of the element pattern by modification of the physical shape of the antenna element may be impractical because the desired element pattern may require an element aperture size which exceeds the necessary element spacing in the array. A practical approach to overcome the physical element size limitation is to provide networks for interconnecting each antenna input port with more than one antenna element, so that the effective element pattern associated with each input port is formed by the composite radiation of several elements.
One prior art approach to this problem has been described by Nemit in U.S. Pat. No. 3,803,625. Nemit achieves a larger effective element size by providing intermediate antenna elements between the primary antenna elements and coupling signals from the primary antenna element input ports to the intermediate elements. This approach is illustrated in FIG. 1 which shows an array of elements 10, 11 which are coupled to input ports 12. The signals supplied to input ports 12 are split by power dividers 13 and supplied directly to primary elements 10 by transmission line 14 and to intermediate elements 11 by transmission lines 16 and power combiner 17. Nemit's approach provides an aperture excitation consisting of three active elements for signals supplied to each of the input ports. When a signal indicated by arrow 18 is supplied to any of the input ports 12, the associated primary antenna element 10 has a large amplitude excitation indicated by arrow 19 and the adjacent intermediate antenna elements 11 have a lower amplitude excitation indicated by arrows 20. This tapered multi-element aperture excitation produces some measure of control over the radiated antenna pattern.
A more effective prior art antenna coupling network is described by Frazita et al. in allowed U.S. pat. application Ser. No. 594,934, filed July 10, 1975 and now U.S. Pat. No. 4,041,501, which is assigned to the same assignee as the present invention. According to the technique of Frazita, illustrated in FIG. 2, the antenna elements 22 are arranged in element modules 20, each of which is provided with an input port 24. Transmission lines 26 and 28 are coupled to all of the antenna element modules 20 in the array and couple signals supplied to any of input ports 24 to selected elements in all the antenna element modules of the array, thereby providing an effective element aperture which is co-extensive with the array aperture. The signals supplied to the elements have a tapered amplitude distribution and periodical phase reversal to approximate an ideal sin x/x aperture distribution which produces a sharply defined sectoral effective element pattern. This technique is an effective and cost efficient method for obtaining substantial control over the effective antenna element pattern for each input port.
It is an object of the present invention to provide an alternate array antenna system having antenna element pattern control by the intercoupling of antenna element modules.